1. Field of the Invention
The invention relates to a process for the production of composite insulators with shielding made of UV-crosslinking silicone rubber.
2. Description of the Related Art
Silicone-elastomer-composite insulators and processes for the production of these are known. Silicone-rubber injection molding using what is known as solid rubber (HTV—high-temperature-crosslinking or HCR—high consistency rubber) features injection of silicone rubber of comparatively high viscosity into heated molds. The process is described by way of example in EP 1091365 for what are known as hollow insulators. The process is currently used for all types of components including, for example, rod insulators and surge arresters. The cycle times, which are sometimes long, have an adverse effect on the process and result from the requirement that the parts to be sheathed (e.g. fiber-reinforced epoxy-resin rods or corresponding tubes), in particular the metallic add-on parts (fittings) that sometimes protrude from the mold, must likewise be heated to the crosslinking temperature of the rubber. Large components sometimes require machines and apparatuses of considerable size.
Another disadvantage is the presence, on the molded component, of mold-parting lines which often require that the moldings be subjected to a subsequent mechanical operation.
A similar process is available using machines for lower pressures and what are known as liquid rubbers (LSR—liquid silicone rubber).
There are somewhat earlier processes, which therefore preceded the availability of large injection-molding machines, and which manufacture shields (DE 2746870) and sometimes the core sheathing (EP 1130605) individually, and then assemble these. Here again, solid rubbers are mainly used. Advantages of the processes are the flexibility of the arrangement of the shields. The large number of operations and the large number of shield-core insulation connection points and/or shield-shield connection points can have a disadvantageous effect.
Solid rubber is likewise used in processes for the production of helical shielding (EP 821373). Although that process is universally applicable, it can have the disadvantage that a connection point is likewise produced between each location and each adjacent location. The process cannot be fully automated.
The early processes, all of which can be termed casting processes (DE 2044179, DE 2519007), require the use of comparatively low-viscosity rubber. They all use what is known as room-temperature-crosslinking 2-component rubber (RTV-2), which when used can be crosslinked at slightly elevated temperature. Because each operation manufactures an individual shield, the process can be used substantially independently of the final size of the component. This technology is therefore currently useful for insulators with very large diameter. There are no longitudinal parting lines requiring a subsequent mechanical operation. A disadvantage is the long cycle time resulting from the comparatively low crosslinking rate of the rubbers used.
A feature common to all of the known processes is that the crosslinking of the electrically insulating material of the exterior sheath of the insulators either occurs spontaneously at room temperature or is initiated thermally at elevated temperature. The crosslinking at room temperature (possible by way of example in the conventional processes with open molds in accordance with DE 2044179 and DE 2519007) requires some ten minutes to some hours, and the crosslinking at elevated temperature requires a period of some minutes to some tens of minutes in the processes using molds (EP 1147525, DE 2746870, and EP 1091365) up to more than 100 minutes in the case of subsequent crosslinking in an oven, e.g. in accordance with processes described in EP 821373 and EP 1130605.